1. Field of the Invention
The present invention relates to a solid electrolyte gas sensor using a solid electrolyte material such as zirconia ceramic.
The present invention also relates to a process of producing a joined body of a ceramic member and a crystal of a hard-to-sinter oxyacid salt such as a sulphate or a carbonate.
The present invention also relates to a process of producing a hard-to-sinter oxyacid salt such as a sulphate and a carbonate joined to a ceramic member.
2. Description of the Related Art
Because of their high ion conductivity, solid electrolyte materials are advantageously used in gas sensors. For example, zirconia ceramic, which has a oxygen ion conductivity sufficient for an oxygen gas sensor, is actually used as a sensor for detecting an oxygen concentration.
FIG. 1 shows an oxygen gas sensor comprising zirconia ceramic, in which a tube 104 of zirconia ceramic has electrodes 105 and 106 on the inner and outer side walls thereof, respectively, to detect an electromotive force between the electrodes 105 and 106 while a selected gas is allowed to flow through the tube 104.
This oxygen gas sensor detects an oxygen gas concentration by measuring an electromotive force generated between both sides of the wall of the zirconia ceramic tube 104 because of migration of oxygen ions through the zirconia ceramic wall when oxygen concentrations are different on both sides of the wall. Therefore, to improve the detecting efficiency of the gas sensor, the ion conductivity must be improved, so that a gas sensor using a solid electrolyte is actually operated while being heated at several hundreds of degrees centigrade.
Thus, a gas sensor using a solid electrolyte material need be heated to an elevated temperature when operated, so that the shown oxygen gas sensor has a nichrome wire heater 107 surrounding and heating the zirconia ceramic tube 104. The numeral "108" denotes a thermocouple. This causes a first problem that a gas sensor is large and is complicated in structure.
Sulphates, carbonates, nitrates, etc., have a poor sinterability and are difficult to form into a sintered body or, if sintered, have a poor durability, although they are used to make various devices including solid state gas sensors.
Moreover, crystals of these salts, when produced by heating to a liquid and subsequent cooling to a solid, involve cracks induced by a change in the crystal structure occurring upon a phase transformation at a specific temperature. Such cracks are detrimental to those devices that need a hermetic seal.
Sulphates, carbonates, nitrates, etc., are easily decompose to form SOx, COx, NOx, etc. at elevated temperatures and decompose by reaction with water at room temperature. Therefore, gas sensors using these salts are too fragile to be applied in practical use.
Nevertheless, there is known a solid state gas sensor having a zirconia ceramic base on which auxiliary electrodes of a hard-to-sinter oxyacid salt such as a sulphate or a carbonate are provided.
Zirconia ceramic has a conductivity for oxygen ions and thus functions as a solid electrolyte when heated at an elevated temperature ranging from about 550.degree. C. to about 1500.degree. C., so that the difference between oxygen gas concentrations prevailing on the both sides of a zirconia ceramic base or substrate induces oxygen ion conduction through the base thickness to generate an electromotive force between opposite sides of the base. A gas sensor operates by measuring the thus-generated electromotive force.
FIG. 2 shows a sensor structure in which a sodium sulphate layer 206 is joined to the upper surface of a zirconia ceramic base 205 and a reference electrode 207 and a detecting electrode 208, both of platinum, are provided on the lower surface of the base 205 and the upper surface of the sodium sulphate layer 206.
This structure is different from the aforementioned gas sensor in that an auxiliary electrode 206 of sodium sulphate is provided on the zirconia ceramic base 205 to detect sulphur dioxide and other sulphur oxides.
To produce the above-recited gas sensors, it is important to form a crystal of a sulphate or a carbonate on a base. However, sulphates, carbonates, nitrates, etc. are not only hard to sinter but also easily decomposed and can hardly be applied in actual devices.
The above-mentioned second problem must be solved in order to form a crystal of a hard-to-sinter oxyacid salt such as a sulphate on, and joined to, a ceramic base such as zirconia ceramic.
There is also a third problem that, when a layer of a sulphate or the like is joined to a base of zirconia ceramic or other ceramic, interaction acting across the joint interface is week because the ceramic base has a smooth surface, so that the joined layer easily exfoliates from the base.
To solve this problem, the base surface is roughened or scratched to provide an increased strength of the joint between a sulphate layer and the base. However, this does not provide a sufficient solution because thermal shocks such as repeated heating and cooling readily cause exfoliation.